Temperature dependence on reaction of CaCO 3 and SO 2 in O 2 /CO 2 coal combustion

Transcription

1 DOI: /s Temperature dependence on reaction of CaCO 3 and SO 2 in O 2 /CO 2 coal combustion WANG Hong( ) 1, 2, XU Hui-bi( ) 2, ZHENG Chu-guang( ) 1, QIU Jian-rong( ) 1 (1. National Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan , China; 2. School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan , China) Abstract: The temperature dependence on the reaction of desulfurization reagent CaCO 3 and SO 2 in O 2 /CO 2 coal combustion was investigated by thermogravimetric analysis, X-ray diffraction measurement and pore structure analysis. The results show that the conversion of the reaction of CaCO 3 and SO 2 in air is higher at and lower at compared with that in O 2 /CO 2 atmosphere. The conversion can be increased by increasing the concentration of SO 2, which causes the inhibition of CaSO 4 decomposition and shifting of the reaction equilibrium toward the products. XRD analysis of the product shows that the reaction mechanism of CaCO 3 and SO 2 differs with temperature in O 2 /CO 2 atmosphere, i.e. CaCO 3 directly reacts with SO 2 at 500 and CaO from CaCO 3 decomposition reacts with SO 2 at The pore analysis of the products indicates that the maximum specific surface area of the products accounts for the highest conversion at in O 2 /CO 2 atmosphere. The results reveal that the effect of the atmosphere on the conversion is temperature dependence. Key words: CaCO 3 ; SO 2 ; O 2 /CO 2 coal combustion; temperature dependence 1 Introduction Extensive research has been devoted recently to CO 2 -capture techniques because of growing concerns with respect to greenhouse gas emissions [1]. CO 2 emission from fossil fuel combustion is a significant source of global warming and climate change. Clean coal combustion technologies have been developed toward easy CO 2 recovery, low NO x emission and high desulfurization efficiency [2 6]. O 2 /CO 2 coal combustion is one of these new technologies associated with reduced CO 2 emission to the atmosphere [7 8]. Ca-based sorbent such as CaCO 3 is used as sulfur absorbent in conventional combustion atmosphere [9 10]. The reaction of desulfurization reagent CaCO 3 and SO 2 has been a subject of increasing attention in O 2 /CO 2 coal combustion [11 12]. LIU et al [13] reported sulfation behavior of limestone at high CO 2 concentrations in O 2 /CO 2 coal combustion. They found that sintering was much mitigated during direct sulfation of limestone. The rate of direct sulfation does not decrease as much as the CaO-SO 2 sulfation. CHEN et al [14] investigated the calcination and sintering characteristics of limestone in O 2 /CO 2 atmosphere. The results showed that the specific pore volume and specific surface area of CaO calcined in O 2 /CO 2 atmosphere were less than those of CaO calcined in air at the same temperature. FUERTES et al [15] studied the direct sulfation behavior of dolomite at a high CO 2 partial pressure and temperatures ranging from 650 to 875. The results suggested that solid diffusion was the control process in direct sulfation of dolomite. However, most of these investigations were carried out by using thermogravimetric analysis (TGA) at temperatures below [16 17]. In O 2 /CO 2 coal combustion, the concentrations of CO 2 and SO 2 in the flue gas may be enriched up owing to the gas recirculation. At high CO 2 and SO 2 concentrations, and high calcium conversion, the influence of high SO 2 concentration on the calcium conversion and the reaction mechanism of desulfurization reagent CaCO 3 and SO 2 are not clear at temperatures ranging from 500 to Therefore, it is necessary to study the calcium conversion and mechanism in O 2 /CO 2 coal combustion at high CO 2 concentration. However, few researches have been reported about the composition of sulfated products and the desulfurization mechanism in the O 2 /CO 2 coal combustion with high concentration of CO 2. Foundation item: Project( ) supported by the National Natural Science Foundation of China; Project(306012) supported by the Key Foundation of Ministry of Education of China Received date: ; Accepted date: Corresponding author: WANG Hong, PhD; Tel: ; hongwzy@mail.hust.edu.cn

2 846 In this work, the calcium conversions in O 2 /CO 2 coal combustion at were investigated and the effect of concentration of SO 2 on calcium conversion was also studied. The reaction mechanism of desulfurization reagents CaCO 3 and SO 2 in O 2 /CO 2 coal combustion was investigated by thermogravimetric analysis, X-ray diffraction measurement and pore structure analysis. 2 Experimental 2.1 Materials O 2, CO 2, N 2 and SO 2, each with mass fraction of 99.9%, were purchased from Wuhan Gas Co. Ltd, Wuhan, China. 99.9% (mass fraction) CaCO 3 was purchased from Sinopharm Chemical Reagent Co. Ltd, Beijing, China. 2.2 Experimental setup Experiments were performed in a fixed-bed quartz reactor placed in a tubular oven. The reactor was 650 mm in length. A sintered quartz filter of 20 mm in diameter was placed at the middle of the reactor to support the sample powder. The temperature in the reactor was measured by a thermocouple placed immediately under the sample. Premixed gases were introduced into the reactor from an inlet above the reactor, and the gas flow rates were regulated by mass flow controllers. The concentration of SO 2 in the premixed gases was measured by a SO 2 sensor (Z 1300, ESC). 2.3 Experimental procedure In the experiments, two kinds of reactive atmospheres were utilized: (1) 21.0% O 2 /79.0% CO 2 with trace SO 2 (designated as O 2 /CO 2 atmosphere) and (2) 21.0% O 2 /79.0% N 2 with trace SO 2 (designated as air atmosphere). The experiments were carried out under reactant gas conditions. In all cases, isothermal conditions in the quartz reactor were first established and the desired reactant gas conditions were then achieved. For each run, 0.5 g of the CaCO 3 powder was rapidly loaded from a sample feeder onto the sintered quartz filter in the reactor. After a specified time period was over, the sample from the fixed bed reactor was allowed to cool down under N 2, and subsequently placed in a desiccator for characterization and analysis. The content 2 of SO 4 in the product was determined by the gravimetric method, and the calcium conversion (η) was calculated by the following equation: m(caso4) η = 100% (1) m(caco ) 3 where m(caso 4 ) is the mass of CaSO 4 in the product, and m(caco 3 ) is the initial mass of CaCO Characterization of reaction product Thermogravimetric analysis was performed with a TG/DSC analyzer (STA 449C, Netzsch). In the TG analysis, the temperature was ramped from room temperature to at a rate of 20 /min and then held at for 10 min under a steady flow of mixed gases of 50 ml/min. The mixed gases were of 21.0% O 2 / 79.0% CO 2 or 21.0% O 2 /79.0% N 2, which were similar to those described in section 2.3, but without SO 2. The starting decomposition temperature was defined as the temperature when the thermogravimetric curve started to deviate from the baseline. The end decomposition temperature was defined as the temperature when the maximum mass loss occurred. The sulfated products were characterized by X-ray diffractometry (XRD, D/max- B, Rigaku) using an X-ray diffractometer with Cu K α radiation (λ= nm) and a Ni ray filter. The tube current and the voltage were set at 30 ma and 30 kv, respectively. The peak positions of samples were compared with JCPDS files. BET surface areas of sulfated products were measured with N 2 adsorption analysis by using a specific surface area analyzer (ASAP 2020, Micromeritics). 3 Results and discussion 3.1 Effect of reactive atmosphere on calcium conversion (η) In conventional air combustion, the reaction of CaCO 3 and SO 2 occurs as follows: CaCO 3 CaO+CO 2 (Decomposition) (2) CaO+SO 2 +1/2O 2 CaSO 4 (Sulfation) (3) The reaction of CaCO 3 and SO 2 is a solid gas reaction. In general, the reaction rate and η are affected by temperature and atmosphere. Fig.1 shows calcium conversion vs sulfation time at varied temperatures in different atmospheres. Obviously, η increases with the increase of reaction time at a given temperature for both reactive atmospheres. The change of η between 500 and 900 is faster than that between 900 and η, however, decreases when the temperature is higher than It is found from Fig.1 that the values of η in air are much higher than those in O 2 /CO 2 atmosphere when the temperature is lower than 900. For instance, at 900 the values of η in air are 23.1% and 26.9% at reaction time of 30 and 40 min, respectively, after the reaction started; whereas the values of η at the same time are only 9.6% and 16.5% in O 2 /CO 2 atmosphere, respectively. A possible explanation involves the competitive reaction of CO 2 and CaO in O 2 /CO 2 atmosphere. In addition, the

3 847 Fig.1 Calcium conversion (η) vs sulfation time at SO 2 volume fraction 0.2%: (a) In O 2 /CO 2 atmosphere; (b) In air amount of CaO decomposed from CaCO 3 in O 2 /CO 2 atmosphere is less than that in air because of the inhibition of the decomposition of CaCO 3 by higher concentrations of CO 2. When the temperature is increased to 1 100, the effect of both atmospheres on η becomes weaker. For instance, the values of η are 25.8% and 32.4% in air and 23.3% and 29.1% in O 2 /CO 2 atmosphere at reaction time of 30 and 40 min, respectively. More CaO from CaCO 3 decomposition at this temperature is produced than that at 900. At 1 200, η in O 2 /CO 2 atmosphere is higher than that in air, which may be related to less sintering of CaO in O 2 /CO 2 atmosphere. This indicates that O 2 /CO 2 atmosphere is beneficial to the removal of SO 2 at high temperatures. 3.2 Effect of SO 2 concentration on calcium conversion In comparison with conventional air combustion, the O 2 /CO 2 coal combustion features recycling of flue gas, resulting in high concentrations of SO 2 and CO 2 in the combustion atmosphere. Generally speaking, increasing SO 2 concentration causes the equilibrium of sulfation reaction to shift toward the products, leading to a higher η. Figs.2 and 3 show the effect of SO 2 concentration on η at 900 and 1 100, respectively. Fig.2 Effect of SO 2 concentration on η at 900 : (a) In O 2 /CO 2 atmosphere; (b) In air As shown in Figs.2 and 3, η increases with increasing SO 2 concentration at either 900 or As mentioned above, high SO 2 concentrations will be better for the equilibrium of sulfation reaction to shift toward the products. A comparison between Figs.3(a) and (b) shows that the increase of η in O 2 /CO 2 atmosphere is faster than that in air during the reaction time period of min. This can be attributed to the inhibition of the decomposition of CaSO 4 [11]. This result further demonstrates that high temperatures are beneficial to the removal of SO 2 in O 2 /CO 2 atmosphere. 3.3 Decomposition processes of CaCO 3 in both atmospheres The decomposition of CaCO 3 was strongly affected by the concentration of CO 2 and temperature [14]. The decomposition process of CaCO 3 in both atmospheres without SO 2 was investigated by TGA in order to determine the effect of the reactive atmosphere on it. The results are listed in Table 1. As shown in Table 1, the mass losses of the samples are rather close to each other in both atmospheres. However, the decomposition of CaCO 3 starts at 884 in O 2 /CO 2 atmosphere, whereas the decomposition takes

4 848 Fig.4 XRD patterns of sulfated products obtained in air at 900 for different reaction times: (a) 5 min; (b) 20 min; (c) 40 min Fig.3 Effect of SO 2 concentration on η at : (a) In O 2 /CO 2 atmosphere; (b) In air Table 1 Effects of atmosphere on decomposition of CaCO 3 Atmosphere Starting temperature of decomposition/ End temperature of decomposition/ Mass fraction of residua/ % O 2 /CO Air place at 758 in air. The starting and end temperatures of the decomposition in O 2 /CO 2 atmosphere are 126 and 55 higher than those in air, respectively. This is likely due to the constraint or the inhibition of decomposition of CaCO 3 at high concentration of CO XRD analysis of sulfated products Possible sulfated products include CaCO 3, CaO and CaSO 4 according to reactions (2) and (3). An XRD technique was used to characterize their phase compositions to clarify the reaction mechanism of CaCO 3 and SO 2 in O 2 /CO 2 atmosphere and in air. Proportions of phase composition were estimated from relative peak intensities. Fig.4 shows the XRD patterns of sulfated products at 900 for different reaction times in air. According to the JCPDS files, the major diffraction peaks of sulfated products CaCO 3, CaO and CaSO 4 appear at 2θ of 29.4, 37.4 and 25.5, respectively. Obviously, the sulfated products contain a significant amount of CaCO 3 and small amount of CaO and CaSO 4 after 5 min of reaction time. When the reaction time is increased to 20 min, the intensity of the major peak of CaCO 3 is smaller than that of either major peak of CaSO 4 or CaO. When the reaction time is increased to 40 min, the intensity of the major peak of CaCO 3 is very low and the intensities of the major peaks of CaSO 4 and CaO become very high. The phase compositions of sulfated products in different atmospheres for varied reaction times are given in Table 2. Table 2 shows that the phase compositions of sulfated products in two kinds of atmospheres are CaCO 3 and CaSO 3 at 500. At this temperature, the decomposition of CaCO 3 does not occur. This is in concord with the TGA result. CaSO 3 is the direct sulfation product of CaCO 3 and SO 2, as shown in Eq.(4), and this compound is too stable to oxidize to CaSO 4 at low temperatures. CaCO 3 +SO 2 CaSO 3 +CO 2 (4) In air, the phase composition of sulfation products at 700 is dependent on the reaction time. The phases are CaCO 3, CaSO 3 and CaSO 4 after 5 min of reaction. As the reaction time increases, CaSO 3 phase disappears due to the oxidation to CaSO 4. CaO is a absent most likely

5 849 Table 2 Phase compositions of sulfated products determined by XRD Sulfated products in O 2 /CO 2 atmosphere Sulfated products in air Temperature/ Time/min Major phase Minor phase Major phase Minor phase CaCO 3 CaSO 3 CaCO 3 CaSO 3 20 CaCO 3 CaSO 3 CaCO 3 CaSO 3 40 CaCO 3 CaSO 3 CaCO 3 CaSO CaCO 3 CaSO 3 CaCO 3 CaSO 3, CaSO 4 20 CaCO 3 CaSO 3, CaSO 4 CaCO 3 CaSO 4 40 CaCO 3 CaSO 4 CaCO 3 CaSO CaO, CaCO 3 CaSO 4 CaO CaSO 4 20 CaO, CaCO 3 CaSO 4 CaO CaSO 4 40 CaO, CaCO 3 CaSO 4 CaO CaSO 4, CaO CaSO 4 CaO CaSO 4 20 CaO CaSO 4 CaO CaSO 4 40 CaO CaSO 4 CaO CaSO CaO CaSO 4 CaO CaSO 4 20 CaO CaSO 4 CaO CaSO 4 40 CaO, CaSO 4 CaO CaSO 4 because CaCO 3 does not decompose at 700. This result is also in accordance with that of TGA. In O 2 /CO 2 atmosphere, CaCO 3 and CaSO 3 phases are found at 700 after 5 min of reaction. This result is quite different from that in air. When the reaction time is increased to 20 min, CaSO 4 phase is produced. The phase composition is the same as that in air after 40 min of reaction. The results indicate that the effect of atmosphere on the phase composition of sulfation products is reaction time dependent. The sulfation in both atmospheres can be described as follows: CaSO 3 +1/2O 2 CaSO 4 (5) According to Table 1, the decomposition of CaCO 3 starts at 758 in air. Therefore, CaO described in Eq.(2) is found in all samples at 900. After 40 min of reaction, only CaSO 4 and CaO are found and the proportion of CaO is more than that of CaSO 4, suggesting indirect sulfation of CaCO 3, as shown in Eq.(3). Likewise, the decomposition of CaCO 3 starts at 884 in O2/CO 2 atmosphere, as shown in Table 1. When the reaction time is increased from 5 to 40 min, the phase compositions of the sulfation products are CaSO 4, CaO and CaCO 3 at 900. In a high CO2 concentration atmosphere, CaO can be consumed in two ways: (1) sulfation reaction of CaO and SO 2, which forms CaSO 4 ; and (2) recarbonation reaction of CaO and CO 2, which forms CaCO 3. If the decomposition rate of CaCO 3 to produce CaO is equal to the consumption rate of CaO, sintering of CaO can be avoided. This can lead to an optimum η. ANTHONY and GRANATSTEIN [18] reported that the recarbonation reaction is faster than sulfation, causing excessive CaCO 3 in the sulfation product. Therefore, indirect and direct sulfation reactions are simultaneously performed to form CaSO 4. The sulfation mechanism can be described by the combination of Eqs.(2) (5). The phase compositions of the sulfation product are CaSO 4 and CaO in both atmospheres at temperatures ranging from to The major phase is CaO, suggesting a typical indirect sulfation process. The sulfation mechanism can be described by the combination of Eqs.(2) and (3) in this temperature range. 3.5 Pore structure analysis of sulfated products It is known that the pore structure of sulfated products plays an important role in the desulfurization reaction. The pore structure is greatly affected by the reaction temperature and atmosphere. The specific surface areas of sulfated products were measured by the N 2 adsorption method. Fig.5 shows the plots of BET specific surface area of sulfated products vs reaction temperature in both atmospheres after 20 min of reaction. A maximum BET specific surface area is found at 900 in air, whereas the surface area is almost independent of temperature in O 2 / CO 2 atmosphere. As seen from Fig.5, the BET specific surface areas of sulfated products in both atmospheres are almost the same in the temperature range from 500 to 700. This can be attributed to the fact that the decomposition of CaCO 3 does not occur at these temperatures. When the temperature is increased to 900, a distinct difference

6 850 is found between the atmospheres employed. According to the result of TGA (shown in Table 1), the decomposition of CaCO 3 is almost completed at 900 in air, but not in O 2 /CO 2 atmosphere. In this case, the decomposed CaCO 3 is a dominant contributor to the BET specific surface area of sulfated products. As the temperature is increased to and above, the BET specific surface area of sulfated products in air decreases to the same level as that in O 2 /CO 2 atmosphere. This result can be attributed to considerable sintering of CaO and the pore plugging of sulfated products. This suggests that the decrease of surface area of reactant accounts for the lower calcium conversion at high temperatures in air. By contrast, the BET specific surface area of sulfated products is less affected by reaction temperature in O 2 /CO 2 atmosphere, and this further confirms that high temperature is beneficial to the capture of SO 2 in O 2 /CO 2 atmosphere. Fig.5 BET specific surface area of sulfated product vs temperature in different atmospheres 4 Conclusions (1) Temperature has a great influence on the calcium conversion in air and in O 2 /CO 2 atmosphere. The calcium conversion in air is higher than that in O 2 /CO 2 atmosphere at temperatures ranging from 500 to 1 100, and it is higher in O 2 /CO 2 atmosphere at The results show that high temperature is beneficial to the removal of SO 2 in O 2 / CO 2 atmosphere. (2) The calcium conversion increases with the increase of concentration of SO 2 due to shifting of the reaction equilibrium and the inhibition of the decomposition of CaSO 4. (3) The reaction mechanism of desulfurization reagents CaCO 3 and SO 2 in O 2 /CO 2 coal combustion dependents on the temperature in O 2 /CO 2 atmosphere. It involves both direct and indirect sulfation reactions. (4) The BET specific surface area of sulfated products is less affected by reaction temperature in O 2 /CO 2 atmosphere, which is responsible for the high desulfurization efficiency in O 2 /CO 2 coal combustion at high temperature. References [1] YANG H Q, XU Z H, FAN M H, GUPTA R, SLIMANE R B, BLAND A E, WRIGHT I. Progress in carbon dioxide separation and capture: A review [J]. Journal of Environmental Sciences, 2008, 20: [2] NAKAYAMA S, NOGUCHI Y, KIGA T, MIYAMAE S, MAEDA U, KAWAI M, TANAKA T. Pulverized coal combustion in O 2 /CO 2 mixtures on a power plant for CO 2 recovery [J]. Energy Conversion and Management, 1992, 33(5/8): [3] KIMURA N, OMATA K, KIGA T, TAKANO S, SHIKISIMA S. The characteristics of pulverized coal combustion in O 2 /CO 2 mixtures for CO 2 recovery [J]. Energy Conversion and Management, 1995, 36(6/9): [4] KIGA T, TAKANO S, KIMURA N, OMATA K, OKAWA M, MORI T, KATO M. Characteristic of pulverized coal combustion in the system of oxygen/recycled flue gas combustion [J]. Energy Conversion and Management, 1997, 38(S): [5] CROISET E, THAMBIMUTHU K V. NO x and SO 2 emissions from O 2 /CO 2 recycle combustion [J]. Fuel, 2001, 80: [6] LIU H, ZAUKANI R, GIBBS B M. Pulverized coal combustion in air and in O 2 /CO 2 mixtures with NO x recycles [J]. Fuel, 2005, 84: [7] ZHENG L, FURIMSKY E. Assessment of coal combustion in O 2 +CO 2 by equilibrium calculations [J]. Fuel Processing Technology, 2003, 81: [8] LIU H, OKAZAKI K. Simultaneous easy recovery and drastic reduction of SO x and NO x in O 2 /CO 2 coal combustion with heat recirculation [J]. Fuel, 2003, 82: [9] CHENG Jun, ZHOU Jun-hu, LIU Jian-hong, ZHOU Zhi-jun, HUANG Zhen-yu, CAO Xin-yu, ZHAO Xiang, CEN Ke-fa. Sulfur removal at high temperature during coal combustion in furnaces: A review [J]. Progress in Energy and Combustion Science, 2003, 29: [10] TARELHO L A C, MATOS M A A, PEREIRA F J M A. The influence of operational parameters on SO 2 removal by limestone during fluidized bed coal combustion [J]. Fuel Processing Technology, 2005, 86: [11] FUERTES A B, FERMANDEZ M J. The effect of metallic salt additives on direct sulfation of calcium carbonate and on decomposition of sulfated samples [J]. Thermochimica Acta, 1996, 276(25): [12] MAO Yu-ru, FANG Meng-xiang, LUO Zhong-yang, WU Xue-cheng, CEN Ke-fa. Calcination and desulfurization of limestone under O 2 /CO 2 atmosphere [J]. Journal of Fuel Chemistry and Technology, 2004, 32: (in Chinese) [13] LIU H, KATAGIRI S, KANEKO U, OKAZAKI K. Sulfation behavior of limestone under high CO 2 concentration in O 2 /CO 2 coal combustion [J]. Fuel, 2000, 79: [14] CHEN Chuan-min, ZHAO Chang-sui, LIANG Cai, PANG Ke-liang. Calcination and sintering characteristics of limestone under O 2 /CO 2 combustion atmosphere [J]. Fuel Processing Technology, 2007, 88: [15] FUERTES A B, VELASCO G, FERNANDEZ M J, ALVAREZ T. Analysis of the direct sulfation of calcium carbonate [J]. Thermochimica Acta, 1994, 242(15): [16] QIU K R, LINDQVIST O. Direct sulfation of limestone at elevated pressures [J]. Chemical Engineering Science, 2000, 55(16): [17] ALVAREZ E, GONZALEZ J F. High pressure thermogravimetric analysis of the direct sulfation of Spanish calcium-based sorbents [J]. Fuel, 1999, 78: [18] ANTHONY E J, GRANATSTEIN D L. Sulphation phenomena in fluidized bed combustion systems [J]. Progress in Energy and Combustion Science, 2001, 27: (Edited by CHEN Wei-ping)